1. Field of the Invention
The invention relates to disk brakes and, more particularly, relates to an electromagnetic disk brake which has at least one friction disk having a braking surface formed entirely from a rubber material. The invention additionally relates to a friction disk usable in such a brake and to a motor braked by such a brake.
2. Discussion of the Related Art
Electromagnetic disk brakes are widely used in a variety of applications such as dynamic brakes for motor input shafts and/or output shafts and as static or "park-and-hold" brakes for motors and the like. A typical electromagnetic disk brake of this type includes a friction disk that is coupled with the shaft so as to be movable axially relative to the shaft but to be rotationally fixed relative to the shaft. In static braking applications, the friction disk is normally compressed between a pressure plate and an axially-movable clapper plate to provide the desired holding action, and brake release is effected by energizing the electromagnet to retract the clapper plate to allow the friction disk to rotate freely. In dynamic applications, the brake is applied while the shaft is rotating, either by energizing an electromagnetic actuator to overcome the force of a return spring or by de-energizing a normally-energized actuator to permit a compression spring to apply the brake. Whether used in static braking applications or dynamic braking applications, the braking surface of the typical friction disk of these brakes is formed from a composite material, often containing a brittle, hard, composite material.
Many electromagnetic disk brakes strive to maximize braking torque while minimizing size, weight, and power requirements. However, the typical composite material friction disk used in these brakes is less than ideal for these purposes for several reasons.
First, the typical brake disk material has a relatively low coefficient of static friction--usually on the order of about 0.3 to 0.5. Relatively strong springs must therefore be used to compress the disk between the clapper plate and the pressure plate with enough force to impose the required braking torque on the associated shaft. The requirement for strong springs, in turn, imposes a requirement for relatively large, high-powered magnetic coils to withdraw the clapper plate against the force of those springs.
Second, traditional composite friction materials have very little flexibility and, hence, cannot conform to the shape of the mating surfaces of the clapper plate and the pressure plate. Hence, in order to maximize available braking torque, it was necessary to machine the surfaces of the clapper plate and the pressure plate that mate with the friction disk to within 0.0005" to 0.0010" of a perfectly flat surface. Obtaining such a high degree of flatness usually requires that the mating surfaces of the clapper plate and the pressure plate be deburred, ground, and burnished. These machining requirements considerably increase the cost of the brake.
Other friction brake materials are known that lack one or more of the disadvantageous characteristics of traditional composite materials, but these other known materials exhibit disadvantages of their own.
For instance, the coefficient of static friction of some materials used in friction disks drops at least 30-50% when the friction disks are heated to temperatures of 100.degree. C. or above. This is problematic because many brakes are heated to temperatures of up to 100.degree. C. during braking due to heat from friction and/or heat transfer from the braked motor or other braked element.